In this sort of lighting apparatus, at least one organic electroluminescence element is driven at low drive voltage (e.g., approximately a few to 10s V) in comparison with discharge lamps. The element is hereinafter also referred to as an organic EL element or an OLED (Organic Light Emitting Diode). Accordingly, since OLED drivers can be produced at low cost in comparison with prior art ballasts, various OLED drivers have been proposed.
For example, Japanese Patent Publication No. 2005-78828 discloses lighting apparatus. The apparatus includes an organic EL element(s), a converting means, a switching means and a controlling means. The converting means is formed of a full wave rectifying circuit and a smoothing circuit, and converts alternating-current power into direct-current power. The switching means has four transistors constituting a full-bridge circuit. The switching means also produces an alternating electric current from the direct-current power to supply the element with a forward direction current and a reverse direction current alternately. The controlling means turns the transistors on and off based on an electric current flowing through the element. For example, the controlling means adjusts a duty ratio with respect to the transistors in accordance with a designated luminance value. Specifically, in case that the luminance is raised, the duty ratio is increased so as to lengthen a supply time of the forward direction current to the element. Conversely, in case that the luminance is lowered, the duty ratio is decreased so as to shorten a supply time of the forward direction current to the element.
Japanese Patent No. 3432986 discloses an organic EL display device. Before applying emission drive voltage to an organic EL element, the device applies auxiliary voltage to the element. The auxiliary voltage is lower than the emission drive voltage and higher than barrier voltage of the element.
In order to realize lighting control (dimming), the apparatus described in Japanese Patent Publication No. 2005-78828 adjusts the duty ratio while keeping the frequency of the full-bridge circuit constant. Accordingly, the circuit structure of the controlling means becomes complicated.